Because that is what those people want to play. There still isn't anything in a similar style, BC2 lacks maps even close to the same size. They play it because they have been playing it, or because of the mods.Easily... It's still got quite an active user base even now.
Take away the familiarity and the mods, not so sure.
If vanilla BF2 launched tomorrow, with new graphics n all, it'd be lacking compared to the competition.